The present invention relates to a method of producing a light absorbing layer of a compound semiconductor thin-film solar cell.
FIG. 1 shows a basic structure of a thin-film solar cell manufactured from a general chalcopyrite compound semiconductor, which comprises a SLG (soda lime glass) substrate 1 on which a back molybdenum (Mo) electrode layer (positive electrode) 2, a light absorbing layer 5, a buffer layer 6 (ZnS, Cds, etc.) and a transparent electrode layer (negative electrode) 7 made of ZnO, Al, etc are sequentially formed in the described order.
In the thin-film compound semiconductor solar cell, the light absorbing layer 4 is made in the form of a CIGS (Copper-Indium-Gallium-Selenium) thin film made of Cu(In,Ga)Se2 of Ib-IIIb-VIb2 group (based on Cu, (In,Ga), Se), which possesses high power conversion efficiency exceeding 18%.
There is known a conventional method of producing a light absorbing layer of CIGS, which is called “selenization method” by which a selenium (Se) compound is produced by thermo-chemical reaction of a thin-film metal precursor with Se supplied from a Se-source such as H2Se gas source.
U.S. Pat. No. 4,798,660 discloses a method whereby a thin metal film formed of a metal back-electrode layer, a pure copper (Cu) single layer and a pure indium (In) single layer sequentially deposited by a DC magnetron sputtering method is selenized in a selenium (Se) atmosphere (preferably in H2Se gas) to produce a light absorbing layer having a homogeneous composition of CIGS (copper indium diselenium).
Japanese Laid-Open Patent Publication No. H10-135495 describes a metal precursor which is a thin metal laminate formed by sputtering first with a target of Cu—Ga alloy and then with a target of pure indium. As shown in FIG. 2, a thin firm of CIGS for a light absorbing layer 4 is formed on a Mo electrode layer 2 deposited on a SLG (soda lime glass) substrate 1. Namely, a Cu—Ga metal thin layer 31 is first deposited on the Mo-electrode layer of the substrate by the first sputtering process SPT-1 using the Cu—Ga alloy target and then an In metal thin layer 32 is deposited on the Cu—Ga layer 31 by the second sputtering process SPT-2 using the In target T2 to produce a metal-laminated precursor 3 which is then treated by heat in the presence of Selenium (Se) gas by a heat treatment process HEAT to obtain a light absorbing film 5 in the form of a thin CIGS film.
However, this precursor 3 being a laminate of a Cu—Ga alloy layer 31 and an In layer 32 may be subjected to solid-state (interlayer) diffusion of elements which react with one another to form an alloy Cu—In—Ga at a boundary between the laminated layers both in the process of forming the precursor and in the state of being temporarily stored. This reaction still progresses during the selenization of the precursor. As it is difficult to evenly control the alloying reaction process between samples (requiring control of parameters relating to the alloying reaction, for example, temperature, time, etc), the quality of the light absorbing layers 5 may considerably vary. The aggregation of indium is apt to occur, resulting in uneven composition in the layer.
Therefore, there has been proposed such a solution that the concentration of Ga in the precursor has a gradient of Mo, which may decrease in the direction toward the surface from the boundary with the Mo electrode layer 2. However, the conventional method still involves a problem that elements Ga segregate on the boundary between the Mo electrode layer 2 and the Cu—In—Ga layer, causing insufficient adhesion between the light absorbing layer 5 of CIGS and the Mo electrode layer 3. This may be a cause of degradation of the performance of the solar cell product.
In the literature Thin Solid Films 361–362 (2000), pages 9–16, it was reported that the conventional solar cells using a CuInSe2 film containing Na components diffused from a soda lime glass of a substrate diffused and grown therein could attain higher power conversion efficiency (see M. BodegÅrd et al.: “Growth of Cu(In,Ga)Se2 thin films by coevaporation using alkaline precursors”).
In the first World Conference on Photovoltaic Electric Power Conversion (“WCPEC”), there have been reported further findings: a CIGS film deposited on the glass containing Na has a small resistance; a solar cell produced by forming CIGS on a Na2O2 film preformed on a substrate can attain power conversion efficiency improved by about 2% as compared with that of a solar cell manufactured without deposition of a Na2O2 film and its power conversion efficiency that generally depends largely on the Cu-to-In ratio can be constant irrespective of the Cu-to-In ratio (M. Ruckhetal, “Influence of Substrates on the Electrical Properties of Thin film of Cu (In, Gs)Se2”, First WCPEC, Dec. 5–9, 1994).
As is apparent from the above reports, the diffusion or addition of Na is effective to promote the growth of the CuInSe2 film, increase the carrier concentration and improve the power conversion efficiency of the solar cell.
Japanese Laid-Open Patent Publication No. H8-222750 discloses as a method of doping Na in the process of selenizing a laminated precursor formed after preparation of a Na layer by sputtering or depositing. The problem with this process is that a layer of Na or Na compound possessing the hydroscopic property may be denatured by exposing to atmosphere, resulting in peeling of the layer in the precursor.
To solve the above problem, U.S. Pat. No. 5,422,304 discloses a deposition method of manufacturing the CuInSe2 film by doping Na together with other elements composing the light absorbing layer. A method of depositing a Cu—In—O:Na2O2 by sputtering on the Mo electrode layer is also disclosed. However, these methods require rather complicated processes.
Thus, in the process of producing a thin film compound semiconductor solar cell by forming a laminated precursor film composed of a Cu—Ga alloy layer and an In layer on a back electrode and treated by heat in a selenium atmosphere to form a CIGS light absorbing layer, the method of improving the power conversion efficiency of the solar cell by diffusing Na in the light absorbing layer involves a problem of peeling of the Na layer by denature.
The doping of Na in the process of selenization and the selenization of the laminated precursor formed on the Cu—In—O;Na2O2 deposited by sputtering on the back electrode have a common problem of complicating the working process.